This invention relates to an amorphous solar cell and more particularly to a multilayer solar cell. Amorphous silicon (herein designated as a-Si:H) formed by glow discharge of SiH.sub.4 gas or deposition or sputtering has been actively used in new devices by reason that its valence electron can be easily controlled. The control of the valence electron is caused by the remarkable decrease of local level density in the band-gas of a-Si:H. Intense interest has been shown recently toward the amorphous silicon layer used in a solar cell because it can be formed with low cost. To achieve high efficiency in the solar cell is a very important subject for future study in the production of the cell.
But it is impossible to increase the efficiency of a conventional solar cell, so that the development of a new method of forming the solar cell is very important.
The a-Si:H layer formed by glow discharge has the possibility of improving the abilities of the solar cell as its properties, such as the diffusion length of minority carriers or the field intensity in a non-doped a-Si:H layer (herein designated as an i layer) are promising. The conventional a-Si:H layer has a band-gap energy of about 1.8 to 1.95 eV. On the other hand, the conventional solar cell uses a silicon layer, the most suitable band-gap energy of which is about 1.5 to 1.6 eV, so that the efficiency of the solar cell with the conventional a-Si:H layer is less than one with Si layer. In the a-Si:H layer, moreover, an i layer with a thickness of 0.4 to 0.6 .mu.m limits the efficiency because of the short minority carrier diffusion length .DELTA.p of about 0.2 to 0.5 .mu.m.
The low light-utilization efficiency is an essential defect of the conventional a-Si:H layer solar cell. It is necessary for an increase in the light-utilization efficiency to develop a new amorphous material with a high light absorption property, i.e., a low optical energy gap (Egopt). But the effective utilization of a low band-gap amorphous material involves the following technical problems:
1. The Egopt value of the a-Si:H layer formed by glow discharge does not depend on growth condition in the discharge method, such as the growth temperature, gas pressure, input gas quantity or FF power, so it is difficult to form an a-Si:H layer with an Egopt value selected from within a wide range in one growing apparatus.
2. An a-Si:H layer including fine silicon crystal particles (herein designated as .mu.c.a-Si:H) is able to assume an Egopt value intermediate the values of a-Si:H and single crystal silicon with an Eg of 1.1 eV. An a-Si:H composition of mixed phase a-Si:H and a-Ge:H has an Egopt which is continuously changable from 1.0.about.1.1 eV to 1.85.about.1.9 eV. But these a-Si:H layers have many ill desired physical properties and are not utilized for solar cells.